Elastomers, gels and resins prepared from curable organosiloxane compositions containing finely divided silver particles exhibit high levels of electrical conductivity and are therefore used in specialized applications that require materials exhibiting heat resistance, flex resistance, and electrical conductivity.
Japanese Patent Application Laid Open [Kokai or Unexamined] Number Hei 3-170,581 [170,581/1991] teaches an electrically conductive silicone rubber composition comprising an organopolysiloxane containing at least 2 alkenyl radicals in each molecule, an organohydrogensiloxane containing at least 2 silicon-bonded hydrogen atoms in each molecule, a platinum-containing hydrosilylation catalyst, and silver particles.
The silver particles used to prepare electrically conductive silicone rubber are typically classified as chemically reduced silver, electrolytically reduced, and atomized silver. Chemically reduced silver is prepared by reacting an aqueous silver nitrate solution with a reducing agent such as hydrazine, formaldehyde or ascorbic acid. Electrolytically reduced silver is prepared from aqueous silver nitrate solutions by electrolytic deposition at the cathode. Atomized silver particles are prepared by spraying molten silver maintained at a temperature of at least 1,000° C. into water or an inert gas.
Silver particles are available in the form of granules, flakes, dendrites or amorphous particles. Silver flake is preferably used because it yields silicone rubbers with particularly high electrical conductivities.
Japanese Laid Open Patent Application No. 59/170,167 discloses a method for preparing gold- or silver-filled inks by blending an organic solvent with a powdered form of gold or silver that has been treated with the combination of a methylhydrogen polysiloxane and an amino-functional silicone oil.
U.S. Pat. No. 5,227,093, which issued on Jul. 13, 1993 teaches increasing the electrical conductivity of elastomers and other products prepared from curable organosiloxane compositions containing finely divided silver particles by treating the silver particles with a fatty acid ester prior to blending the sivler particles with the other ingrediets of the curable composition.
Japanese Laid Open Patent Application No. 03/49,105 describes electrically conductive particles suitable for use with adhesives. The particles exhibit diameters of from 1 to 20 microns and comprise a core of a high polymer on which is deposited a layer of silver followed by a layer of gold. During blending with the ingredients of a curable organosiloxane composition the particles are treated with a silane coupling agent selected from the group consisting of gamma-methacryloxypropyltrimethoxysilane, gamma-mercaptopropyltrimethoxysilane and gamma-chloropropyltrimethoxysilane.
Several problems are associated with the electrically conductive silicone rubber compositions taught in Japanese Patent Application Laid Open No. Hei 3-170,581 and the other publications mentioned in the immediately preceding paragraphs. When silver in flake-form is used as a filler, it has been found that during storage of the composition not only do the silver particles separate from the composition, but the curability of the composition declines with the passage of time to the point that the composition ultimately becomes uncurable.
Another problem associated with the prior art conductive silicone rubber compositions is the large variations in contact resistance and volume resistivity that occur with the passage of time in the cured elastomers prepared using the curable compositions described in this patent publication. This phenomenon renders the rubber unsuitable for the continuous electrical connection of electrically conductive elements.
The present inventors have been able to determine that one cause of the variation in electrical properties of silver-filled organosiloxane elastomers with time is the low affinity of silver particles for the other ingredients of the curable composition used to prepare the elastomer.
The present inventors have also been able to confirm that the curability of electrically conductive silicone rubber compositions declines with elapsed time due to the presence of residues of the lubricant that is present during grinding of the silver particles. These lubricant residues remain on the surface and/or in the interior of the particles.
One or more of the following lubricants typically have been used with silver particles during grinding of the particles: saturated and unsaturated higher fatty acids such as lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, arachidic acid, and behenic acid; metal soaps such as aluminum laurate, aluminum stearate, zinc laurate, and zinc stearate; higher aliphatic alcohols such as stearyl alcohol; the esters of higher aliphatic alcohols and carboxylic acids; higher aliphatic amines such as stearylamine; higher aliphatic amides; and polyethylene waxes.
The present inventors attempted to remove the lubricant on the surface of lubricant-treated silver flake by subjecting the silver particles to repeated washings with organic solvents. The washings did not provide a satisfactory suppression of the timewise variation in the curability of electrically conductive silicone rubber compositions containing this type of silver as a conductive filler.
The present inventors have also confirmed that a decline in adhesion and affinity between the cured silicone elastomer and the silver particles contribute to timewise variations in, respectively, the contact resistance and volume resistivity of electrically conductive silicone rubbers prepared by curing silver-filled electrically conductive silicone rubber compositions.
As the result of extensive investigations directed at solving the problems described in the preceding paragraphs of this specification, the present inventors discovered that the timewise variation in curability can be suppressed by treating the finely divided silver particles with an organosilicon compound prior to combining the particles with the other ingredients of the curable organosiloxane composition.
The present inventors also discoverd that the presence in the curable organosiloxane compostion of an organosilicon compound containing silicon-bonded alkoxy groups that is in addition to the compound used to treat the silver particles, further reduces the timewise variations in contact resistance and volume resistivity of the cured elastomer. The present invention is the result of these two discoveries.